The development of laboratory tests for hepatitis A virus and hepatitis B virus led to the discovery that there is one or more additional agent(s) responsible for transfusion-associated viral hepatitis. Non-A, non-B disease occurs in a significant proportion of recipients of whole blood and blood components that connot be sterilized by heat treatment. Thus, although the screening of donors for carriers of hepatitis B virus has significantly reduced the frequency with which that etiologic form of transfusion-transmitted disease occurs it has had no impact on the frequency of the non-A non=B agent(s). The most feasible approach to prevention at the present time would appear to be dilution of the virus(es) to non-infectious levels by use of specially treated erythrocyte suspensions. Experience with glycerolization-deglycerolization-washing suggests little likelihood of infectivity of erythrocyte suspensions prepared in this way, but the observations are almost entirely uncontrolled and non-prospective. In instances in which long-term preservation is not needed, simple washing techniques to prepare buffycoat-poor red cell suspensions may be sufficient. A prospective study of recipients of whole blood and washed erythrocytes is proposed to determine the effectiveness of the latter.